Since the early 1970's there has been a strong interest in synthesizing alkali metal trisubstituted borohydride reagents because of their unique reactivity and synthetic utility in organic chemistry. Alkali metal trisubstituted borohydrides reagents are used in organic synthesis primarily as regioselective and stereoselective borohydride reducing agents. To date, all preparations of alkali metal trisubstituted borohydrides have focused on the reaction of alkali metal hydrides or lithium aluminum reagents and trisubstituted boranes to produce the above mentioned compounds. The most common method of producing trisubstituted borohydrides is via the reaction of alkali metal hydrides with trisubstituted boranes as represented by the following equation wherein M represents an alkali metal: ##STR1##
Unfortunately, current processes for the production of trisubstituted borohydride reagents are very limited in scope and have substantial drawbacks associated therewith. For example, commercially available alkali metal hydrides (particularly, lithium and sodium hydrides) are not highly reactive in such reactions, typically requiring special washing and activating steps. Moreover, excesses of the alkali metal hydride are required is such reactions. Even with washing and activating procedures, however, certain trisubstituted borohydrides (for example, lithium tri-sec-butylborohydride) cannot be produced from commercially available alkali metal hydrides. Moreover, other special handling techniques, including hazardous material precautions, are required for alkali metal hydrides (particularly, potassium hydride). Indeed, because potassium hydride reagents are extremely difficult/hazardous to handle, there are no industrial organic processes incorporating such reagents into their synthetic schemes. Furthermore, current procedures for commercial synthesis of alkali metal hydride reagents require relatively high pressures and high temperatures.
It is desirable to develop methods of producing alkali metal substituted borohydride reagents that lessen or eliminate the above drawbacks.